The long-term objectives of this proposal are to develop a paradigm to characterize functional Magnetic Resonance Imaging (fMRI) variation across populations and to design methods to minimize the effects of variation on data analyses. Within the past 10 years, fMRI became one of the most popular ways to noninvasively study brain activity. It has the potential to be one of the best tools to understand neural changes, diagnose diseases, and assess treatments in humans. Still, researchers' abilities to distinguish changes in neural activity from the blood oxygen level dependent changes measured by fMRI limit the technology's potential. The research outlined in this proposal addresses the causes of these limits and brings fMRI closer to becoming a powerful clinical tool. To accomplish these objectives, the first specific aim of this proposal is to quantify fMRI signal variations in young subjects. Subjects will perform breath holding and simple cognitive tasks to examine how fMRI signals change and what variations are due to vascular changes. The second specific aim will extend these studies to compare variations across older adults and subcortical stroke patients. The third specific aim is to include the observed variations in simulations both to assess their statistical ramifications and to design analysis methods that account for variability. This grouping of cognitive tasks and simulations will provide ways to quantify variation across populations and decide how to best use the additional information to design accurate cognitive neuroscience and clinical studies.